U.S. patent application number 11/989184 was filed with the patent office on 2009-09-17 for orthogonal fluxgate magnetic field sensor.
This patent application is currently assigned to Liaisons Electroniques-Mecanques LEM S.A.. Invention is credited to Pavel Kejik, Ozge Zorlu.
Application Number | 20090230955 11/989184 |
Document ID | / |
Family ID | 36001158 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090230955 |
Kind Code |
A1 |
Kejik; Pavel ; et
al. |
September 17, 2009 |
Orthogonal Fluxgate Magnetic Field Sensor
Abstract
Orthogonal fluxgate sensor for measuring an external magnetic
field Hext, comprising a conductor for carrying an excitation
current lexc, a ferromagnetic material adapted to saturate in the
presence of a magnetic field generated by the excitation current,
and at least one pick-up coil adapted to detect variations in the
magnetic field in the vicinity of the magnetic material. The
excitation conductor comprises a substantially linear elongated
portion of conductive, non-magnetic material, forming an excitation
rod (6). The magnetic material surrounds the excitation rod in the
form of a cladding (8).
Inventors: |
Kejik; Pavel; (Ecublens,
CH) ; Zorlu; Ozge; (Lausanne, CH) |
Correspondence
Address: |
BAKER & DANIELS LLP
300 NORTH MERIDIAN STREET, SUITE 2700
INDIANAPOLIS
IN
46204
US
|
Assignee: |
Liaisons Electroniques-Mecanques
LEM S.A.
Plan-les-Ouates
CH
|
Family ID: |
36001158 |
Appl. No.: |
11/989184 |
Filed: |
July 17, 2006 |
PCT Filed: |
July 17, 2006 |
PCT NO: |
PCT/IB2006/002007 |
371 Date: |
January 22, 2008 |
Current U.S.
Class: |
324/253 |
Current CPC
Class: |
G01R 33/04 20130101;
G01R 15/185 20130101 |
Class at
Publication: |
324/253 |
International
Class: |
G01R 33/04 20060101
G01R033/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2005 |
EP |
05405451.5 |
Claims
1. Orthogonal fluxgate sensor for measuring an external magnetic
field H.sub.ext comprising a conductor for carrying an excitation
current I.sub.exc, a saturable magnetic material adapted to
saturate in the presence of a magnetic field generated by the
excitation current, and at least one pick-up coil (10) adapted to
detect variations in the magnetic field in the vicinity of the
magnetic material, wherein the excitation conductor comprises a
substantially linear elongated portion of conductive, non-magnetic
material, forming an excitation rod (6), the magnetic material
surrounding the excitation rod in the form of a cladding (8).
2. Sensor according to claim 1, wherein the ferromagnetic cladding
is deposited around the excitation rod and in direct contact
therewith.
3. Sensor according to claim 1, wherein the pick-up coil,
ferromagnetic cladding, and excitation conductor are built up on a
substrate (16, 16a) in layers formed by deposition and
patterning.
4. Sensor according to claim 3, wherein the sensor further
comprises a circuit for driving the excitation current and
processing the pick-up coil signals, integrated in the
substrate.
5. Sensor according to claim 4, wherein the processing circuit and
pick-up coils are integrated in a CMOS process, on which the
ferromagnetic cladding and excitation rod are built up.
6. Sensor according to claim 1, wherein the pick-up coil comprises
at least two layers of planar spirals (12a, 12b) stacked one of top
of the other.
7. Sensor according to claim 1, wherein the pick-up coil comprises
at least two coil portions, one at each longitudinal end (14) of
the ferromagnetic cladding.
8. Sensor according to claim 7, wherein centers of the respective
pick-up coil portions are positioned adjacent said longitudinal
ends of the ferromagnetic cladding.
Description
BACKGROUND
[0001] The present invention relates to an integrated orthogonal
fluxgate magnetic sensor and method of the fabrication thereof.
[0002] There is generally a need in many applications for a
low-cost miniature magnetic field sensor to measure a weak magnetic
field with certain precision. Fluxgates are the most popular, high
sensitivity magnetic sensors built using an easily saturable
ferromagnetic core. Fluxgate type magnetic sensors may be used for
magnitudinal and directional measurement of DC or low-frequency AC
magnetic fields. Typical applications are electronic compasses,
current sensors, magnetic ink reading, detection of ferrous
materials, and non-destructive testing [1, 2]. The main advantage
of fluxgate sensors are their high sensitivity and very low offset.
On the other hand, low magnetic field operation range and high
perming are problems in current fluxgate sensors [3].
[0003] The working principle of fluxgate sensors based on the
periodic saturation of ferromagnetic material with an AC excitation
field and to detect the change in the flux passing through the
core, which is proportional to the external magnetic field. Two
kinds of configurations in fluxgates are generally known: a
parallel fluxgate, having the excitation field parallel to the
measured field, and an orthogonal fluxgate with the excitation
field perpendicular to the measured field.
[0004] In order to simplify the manufacturing, only one
ferromagnetic core combined with coils can be used as a fluxgate
sensor. When only one ferromagnetic core is used, the orthogonal
fluxgate configuration is preferable because of better signal
treatment dynamic of the fluxgate sensor. The magnetic field to be
measured is physically decoupled from the excitation field by
placing the sensing coil to the orthogonal position with respect to
the excitation field. The contribution of the excitation field is
then removed from the measured signal [6].
[0005] FIG. 1 illustrates an orthogonal configuration of a
conventional single-core fluxgate sensor. The circular excitation
magnetic field Hexc is orthogonal to the axis of the core (C) and
consequently to the external applied magnetic field Hext. Such an
arrangement decouples the sensing coil (S) from the excitation coil
(E).
[0006] The conventional orthogonal fluxgate sensor shown in FIG. 1
is however costly to manufacture, particularly in view of the need
to wind the excitation coil around the ferromagnetic core, a
portion of the coil being fed through the central cavity provided
in the core. Winding of the sensing coil around the core is also
not particularly cost effective. The excitation coil winding also
limits the practical length thereof. The construction also limits
the degree of miniaturization and the possibilities of integration
of the sensor in miniaturized electronic devices.
[0007] In view of the foregoing, several technologies have been
developed in order to integrate the fluxgate sensor in a compact
and cost effective arrangement. The planar configurations as
described in [7], [8] or [9] use always an open-core structure in
parallel configuration and differential mode, with the following
characteristics: [0008] two ferromagnetic cores or two parts of one
ferromagnetic core, [0009] magnetized in longitudinal direction of
the core, [0010] two cores magnetized in opposite
directions--differential mode, [0011] core dimensions (core length
over core section area) and magnetic properties of the core
determinate at once the sensor resolution and the requirements on
excitation field performances, [0012] the excitation of the core
and/or the detection of the measured field is performed by the 3D
micro machined coils enclosing the core, or planar coils situated
under the core and fabricated by using the metallization in CMOS
technology.
[0013] Such known configurations of integrated fluxgate sensors,
while being compact and cost effective to manufacture in large
series, have the following drawbacks: [0014] The use of two
ferromagnetic cores relatively complicates the sensor configuration
and occupies more space. [0015] The open-core structure is not
magnetized thoroughly along the whole length of the core. The core
center saturates first, the tips of the core as the last. The tips
of the core never achieve the deep saturation and it creates
perming of the sensor (i.e. memory effect of the sensor to the hard
magnetic shocks). [0016] Limitation of the core dimensions for
detection and excitation fields. The shorter the core, the wider
the measuring range is at variance with the excitation condition,
however the shorter the core, the more difficulty there is to
generate a magnetic field with sufficient strength to saturate the
core along the whole length. Therefore, the core length influences
in opposite directions the measuring range and the excitation
arrangement. [0017] The use of planar coils for both excitation and
detection leads to a closely coupled coil structure. Such a
structure creates huge capacitive and magnetic parasitic signals
compared to the measured signal. This disturbing signal
deteriorates the sensor signal to noise ratio and stability.
SUMMARY OF THE INVENTION
[0018] An object of this invention is to provide an orthogonal
fluxgate sensor that is compact, cost effective to manufacture in
industrial series, and that demonstrates good performance, in
particular a wide measuring range, low perming effect, and high
precision.
[0019] It would be further advantageous to provide an orthogonal
fluxgate sensor that can be cost effectively integrated in an
integrated circuit and that is easy to drive and control.
[0020] It would be advantageous to provide an orthogonal fluxgate
sensor that has low power consumption.
[0021] Objects of this invention have been achieved by providing
the orthogonal fluxgate magnetic field sensor according to claim
1.
[0022] Disclosed herein is an orthogonal fluxgate magnetic field
sensor comprising an excitation conductor made of a non-magnetic
conducting material, a saturable magnetic material, and at least
one pick-up coil, wherein the excitation conductor comprises an
essentially linear portion in the form of a rod, clad therealong
and therearound by the saturable magnetic material. The one or more
pick-up coils are preferably arranged adjacent ends of the
ferromagnetic cladding. The ferromagnetic cladding may be in direct
contact with the excitation conductor without insulation
therebetween.
[0023] Preferably, the ferromagnetic cladding and excitation
conductor may be formed by deposition and etching of layers that
are built up to form an integrated structure, such as a LiGA
process with copper electroplating and etching for non-magnetic
conductor formation, and ferromagnetic permalloy deposition for the
cladding. The layers may be built up on a silicon substrate in
which the sensor drive and control electronic circuit is also
integrated, such as a case of a CMOS process.
[0024] Advantageously, the sensor configuration according to the
invention significantly decreases the sensor size and footprint and
can be manufactured by integrated circuit layer build-up processes,
whereby the sensor drive and signal processing circuitry may be
directly provided in the semiconductor substrate to provide a very
compact, low cost and low power consuming magnetic field
sensor.
[0025] A further important advantage of the sensor according to the
invention is the even saturation of the ferromagnetic cladding
surrounding the center excitation conductor over the entire length
of the cladding. In addition, the length of the center excitation
conductor and cladding can be varied easily from one sensor design
to another, in order to change the sensitivity and operating range
of the sensor without any significant changes to the manufacturing
process.
[0026] It is also advantageous that the excitation conductor and
sensing coils are separately driven, and independent, thereby
ensuring high sensor signal to noise ratio and stability.
[0027] A single conducting rod going through the center of the
saturable ferromagnetic cladding drastically decreases the required
semiconductor surface occupied by the excitation mechanism,
compared with prior integrated fluxgate sensors containing planar
excitation coils.
[0028] An excitation current I.sub.exc, fed via electric contacts,
for example realized by electroplating and flowing along the center
conducting layer, produces a circular magnetic field H.sub.exc. The
longitudinal magnetic permeability (i.e. in the direction parallel
to the measuring field) is therefore modulated by the circular
magnetic field H.sub.cir, perpendicular to the measuring field. Two
sensing elements represented by the pick-up coils or alternatively
by Hall plate elements positioned below opposite longitudinal ends
of the ferromagnetic cladding are used for the detection of an
alternating magnetic field that is proportional to the external
magnetic field H.sub.ext.
[0029] The cross-section shape of the ferromagnetic cladding and
the central excitation conductor is preferably square or circular,
but may have other forms.
[0030] In order to keep the level of excitation current low and
saturate all parts of the ferromagnetic core, the perimeter of the
ferromagnetic core is preferably reduced down to minimum size
allowed by the layer deposition technology. The center of the each
portion of pick-up coil is located under the longitudinal end of
the ferromagnetic cladding where the field concentration in the
direction perpendicular to the substrate is the highest.
[0031] The ferromagnetic cladding, the sensing coil and the
substrate are isolated from each other by non-conducting,
non-magnetic material. The electrical isolation between the central
excitation conductor and the ferromagnetic cladding is not
necessary, but could be provided, if desired.
[0032] High linearity of the microsensor can be achieved by
introducing a feedback configuration to the system. Either planar
coils positioned under the ferromagnetic structure or an external
discrete coil around the microsensor is the element generating the
compensation field. The planar coil for the feedback can be the
same as that for the signal sensing.
[0033] This magnetic sensor can be combined with an electronic
circuit made in CMOS technology. The electronics may advantageously
provide all the necessary signals for excitation, signal detection
and feedback.
[0034] The different direction of the circular excitation H.sub.cir
from the longitudinal measured field H.sub.ext, to which the
cylindrical ferromagnetic structure is exposed, is very
advantageous. First, since the orthogonal configuration of fluxgate
sensor is used, the contribution of the excitation field is removed
from the measured signal. Secondly, in case of circular excitation
the magnetic structure has a closed path and the ferromagnetic
cladding layer with apparent permeability .mu.a equal to intrinsic
permeability .mu.i is directly exposed to the circular excitation
field H.sub.cir. Consequently, the level of the excitation field
H.sub.cir necessary to reach the cladding saturation stays
unchanged while changing the length I of the ferromagnetic
cladding. Therefore, the length I of the structure has no impact on
the excitation mechanism. Since an external magnetic field in
longitudinal direction is applied, the effect of the length I has
to be taken into account and the demagnetizing factor Nd [5]
determined by structure dimensions tends to reduce the apparent
permeability .mu..sub.a of the structure in longitudinal direction.
The measuring range of the sensor can thus easily be adjusted by
simply changing the excitation conductor and cladding structure
length I.
[0035] The excitation rod is coated with the ferromagnetic material
in a way that a circular magnetic excitation field loop is formed
inside the ferromagnetic material. The permeability of the
ferromagnetic layer is periodically modulated in the orthogonal
direction with respect to the measured magnetic field by passing an
AC current through the excitation rod. This makes the detection
part independent of the excitation mechanism and the measuring
range and the sensitivity can be adjusted by modifying only the
cladding length. The modification of the core length changes the
demagnetization factor [5] of the core. The apparent relative
permeability .mu.app of the core deviates from its intrinsic value
according to the demagnetization factor. This can be expressed
as:
.mu. app = .mu. i 1 + N ( .mu. i - 1 ) ( 1 ) ##EQU00001##
where .mu..sub.i is the intrinsic relative permeability and N is
the demagnetization factor. A change in the apparent permeability
produces a change in the linear region of the B-H curve of the
ferromagnetic cladding layer. Since the saturation magnetic filed
intensity B.sub.sat remains constant, the slope of the linear
region decreases and the magnetic field H required to saturate the
material increases. This corresponds to a decrease in the
sensitivity, but also to an increase in the linear operating range
of the sensor. This phenomenon is effective in the longitudinal
direction in the sensor, i.e. in the sensing direction of the
magnetic field.
[0036] On the other hand, in the radial direction, i.e. in the
excitation direction the geometry can be considered as an
infinitely long magnetic material core due to the closed magnetic
loop, for which the demagnetization factor is zero. So, the slope
of the B-H curve is a maximum, which allows easy and homogenous
saturation along the whole cladding length. The preferably two
planar coils, one positioned under each extremity of the
ferromagnetic cladding, pick up the measured signal. The use of
planar pick-up coils advantageously provides easy integration into
standard CMOS processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a view in perspective of a prior art orthogonal
fluxgate magnetic field sensor;
[0038] FIG. 2a is a view in perspective of an embodiment of an
orthogonal fluxgate magnetic field sensor according to this
invention;
[0039] FIG. 2b is an exploded cross-sectional view through lines
IIb-IIb of FIG. 2a;
[0040] FIG. 2c is a detailed view in cross-section of a central
excitation conductor and ferromagnetic cladding of the sensor
according to the invention;
[0041] FIG. 3 is an exploded perspective view of an embodiment of
an orthogonal magnetic fluxgate according to this invention;
[0042] FIG. 4a is a circuit block diagram illustrating a circuit
for driving the excitation current and processing the sensor
pick-up signals;
[0043] FIG. 4b is a circuit block diagram illustrating a circuit
for driving the excitation current and for processing the sensor
pick-up signals, with feed-back, according to another embodiment of
this invention; and
[0044] FIG. 5 shows, in cross-section, the successive steps (a) to
(l) of build-up of layers to manufacture a portion of the sensor
according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0045] Referring to the Figures, in particular FIGS. 2a to 2c, an
orthogonal fluxgate magnetic field sensor 2, according to this
invention, comprises an excitation conductor 4 having an
essentially linear portion 6, hereinafter referred to as
"excitation rod" clad with a saturable ferromagnetic cladding 8,
and a sensor pick-up coil 10 having coil portions 12 arranged
adjacent opposed ends 14 of the saturable ferromagnetic cladding 8.
The sensor further comprises a substrate 16 on which the cladding 8
and excitation conductor 4 are mounted.
[0046] The substrate 16 may be made of a ceramic, glass or other
insulating material. Preferably however, the substrate 16 is made
of a semi-conductor material comprising an integrated circuit
including the pick-up coil 10 and connected to the excitation
conductor 4. Exemplary embodiments of circuits that may be
integrated in the semiconductor are illustrated in the block
diagrams of FIGS. 4a and 4b, that will be described further on.
[0047] In an embodiment where the substrate 16 acts as a support
without integrated circuit, the sensing coil and excitation
conductor may be provided with respective conducting pads 18, 20
(see FIG. 2a) for bonding to electrical conductors connected to an
external signal processing and excitation circuit. Where the
substrate 16 comprises an integrated circuit, as schematically
illustrated by the layer 16a in FIG. 3, ends 20 of the excitation
conductor may be directly interconnected to the circuit by vias 21
created during layer deposition of the conductor. Likewise, the
interconnections of the pick-up coil between different layers,
respectively to the circuit, may be performed through vias or
contact points 23.
[0048] The substrate 16 with integrated circuit 16a may be made of
other known semiconductor technologies, such as CMOS, in which case
the semiconductor layer with integrated circuit and metallic
conducting layers forming the pick-up coils could be directly
integrated in the substrate resulting in an integrated circuit
composed of 16a, 15 and 13, the magnetic material and excitation
conductor being deposited thereon. An example of the manufacturing
procedure will be described further on.
[0049] The sensing coil 10 is preferably planar, whereby the coil
portions 12 have their centers approximately situated at respective
ends 14 of the ferromagnetic cladding 8. This ensures the maximum
pick-up of the external magnetic field orthogonal to the plane of
the substrate due to the bending of the external magnetic field
lines into the ferromagnetic cladding, the concentration being
greatest at the extremities 14.
[0050] In order to increase the pick-up sensitivity, each coil
portion 12 may be formed of two sets of spirals 12a, 12b provided
on two stacked layers 13, 15, as shown in FIG. 3, thereby doubling
the number of turns of the sensing coil. Since two layers would
normally be required for the sensing coil in order to route the
connection points 18 outside of the center of the coils for
practical purposes, the additional coil does not add any layers and
therefore does not complicate the manufacturing process. Yet
further layers could be added, if greater measuring sensitivity is
required.
[0051] The cladding 8 is preferably formed of a ferromagnetic
material such as a material commonly known as "permalloy", although
other soft magnetic materials that are easily saturable and that
have low hysteresis may be used. The advantage of ferromagnetic
materials is that they are easy to deposit in an advantageous
manufacturing method for building the sensor according to this
invention. The material used for the excitation conductor portion
and the pick-up coil is preferably copper, or copper alloy,
although other non-magnetic materials with good conductive
properties may be used.
[0052] Referring to FIG. 4a, a first embodiment of the signal
processing circuit operating in direct mode will now be briefly
described. A clock signal Clk with a frequency f.sub.o, is
generated by a generator GEN. The frequency of the signal is
divided by 2 and the signal is converted by the driver into a
voltage waveform having only odd harmonics and zero offset such as
a sinusoidal, triangular, or pulsed waveform. This voltage waveform
is converted into a current waveform and fed to the excitation rod
of the ferromagnetic core. The level of the current waveform must
be high enough to saturate the ferromagnetic core. This periodic
saturation produces an induced voltage proportional to the external
magnetic field H.sub.ext, across the terminals of the pick-up coils
placed below the edges of the core. The induced voltage is
amplified and synchronously rectified by the demodulator DEM. Then
the resulting signal is fed to a low pass filter with cut-off
frequency lower than f.sub.o. The output of the low pass filter is
a DC voltage proportional to the external magnetic field H.sub.ext
within the measuring range of the sensor.
[0053] Referring to FIG. 4b, a second embodiment of the signal
processing circuit operating with feedback will now be briefly
described. A clock signal Clk with a frequency f.sub.o, is
generated by a generator GEN. The frequency of the signal is
divided by 2 and the signal is converted by the driver into a
voltage waveform having only odd harmonics and zero offset such as
a sinusoidal, triangular, or pulsed waveform. This voltage waveform
is converted into a current waveform and fed to the excitation rod
of the ferromagnetic core. The level of the current waveform must
be high enough to saturate the ferromagnetic core. This periodic
saturation produces an induced voltage proportional to the external
magnetic field H.sub.ext, across the terminals of the pick-up coils
placed below the edges of the core. The induced voltage is
amplified and synchronously rectified by the demodulator DEM. Then
the signal is integrated in an integrator and passed through a
voltage to current converter. The resulting current is fed to the
feedback coil which produces a compensating magnetic field
H.sub.comp in the opposite direction of the external magnetic field
H.sub.ext, resulting in a decrease in the induced voltage across
the terminals of the pick-up coils. When the system is stable, the
current passing through the feedback coil produces the same level
of the magnetic field as the external field, so that the total
field around the core is zero and no voltage is induced across the
pick-up coils. This makes the outputs of the amplifier and the
demodulator also zero, and the output of the integrator is a DC
voltage proportional to the external magnetic field H.sub.ext.
[0054] FIG. 5 illustrates the sequence of steps in the build-up of
layers in an example of the process steps in manufacturing a sensor
according to the invention. The left hand section of FIG. 5 shows a
representative longitudinal cross-section through the sensor
layers, whereas the right hand sight is representative of a
transverse cross-section.
[0055] In the first step (a), a lower layer of the pick-up coils
12a and contact pads 20 for the excitation rod are formed on the
substrate by known metal deposition techniques such as sputtering,
followed by photolithography and etching to form the pattern,
whereby the deposited metal material is for example an aluminum
alloy AlSi1%.
[0056] Subsequently, in step (b), a contact layer is deposited, for
example SiO.sub.2, by sputtering, followed by photolithography and
etching, to form the patterns, in particular to free the vias 21,23
for interconnection of the two layers of the pick-up coil 12a, 12b,
the second coil layer 12b being deposited in step (c) by the same
techniques as in step (a).
[0057] In step (d), an insulating layer, for example SU-8, is
deposited by spinning, followed by photolithography over which a
seed layer for electroplating is deposited in step (e), for example
Cr/Cu by evaporation.
[0058] In step (f), a mold [4], for example AZ9260, is deposited on
the layers by spinning followed by photolithography, leaving
exposed the openings required for forming a base portion 8a of the
ferromagnetic cladding, for example Permalloy (Fe.sub.19Ni.sub.81),
which is deposited by electroplating and the mold removed as shown
in step (g).
[0059] In step (h), a further mold is is deposited by spinning
followed by photolithography to allow formation of the conductive
excitation rod 6 by electroplating copper and subsequently removing
the mold as shown in step (i). The excitation rod is connected to
vias 21 for connection to the drive circuit.
[0060] In the subsequent step (j), a mold is is deposited by
spinning followed by photolithography, around the partially formed
ferromagnetic base layer 8a and the excitation rod 6 to allow
electroplating of the remaining part of the ferromagnetic cladding
8 around the excitation rod, as shown in step (k). In the last step
(l), the seed layer remaining exposed is removed by etching.
[0061] In case CMOS technology is used, the pick-up coil may
already be integrated in the integrated circuit, in other words,
the equivalent of steps (a) to (d) would already be integrated in
the CMOS fabrication technology, the additional process steps
starting from creation of the seed layer from step (e) onwards.
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